Physical disability is an important outcome from chronic disease in older people. It is associated with morbidity and mortality and adversely affects quality of life for the individual and increases health care costs for society. Despite the public health importance of physical disability, there has been relatively little research examining mechanisms and natural history of physical disability. Osteoarthritis of the knee is a common disorder in the elderly and often leads to physical disability. In this study we propose to examine the association of physical, psychological, social and environmental factors with severity and progression of physical disability from knee osteoarthritis. We will perform a prospective, epidemiological study of 500 elderly people who have painful knee osteoarthritis and physical disability. The subjects will have an assessment of physical disability, both by performance measures and self report, at baseline and every ten months for a total of 30 months. Measurements will be made of disease severity (by magnetic resonance imaging), comorbid disease, strength, aerobic power, gait, pain, health habits, environmental barriers, psychological factors and social support at various time points throughout the study. We will examine the effects of these predictor variables on the severity and progression of physical disability from knee osteoarthritis. The knowledge gained from this study will be used to design interventions aimed at promoting independent functional status and limiting disability in older people with knee OA and to better understand the pathogenesis of disability from chronic disease.